A new housing development near Burgess Hill could cause neighbouring homes to flood, campaigners have claimed.
Developers are eyeing a 460-acre stretch of farmland to the west of the town as a possible site for an estate of 3,500 homes.
But protesters say concreting the countryside would cause a "catastrophic" surge in flooding upstream, because groundwater would have nowhere to drain away.
Christopher Maidment, who organised a protest rally against the proposals in February, said: "Parishes south of the A272 have serious concerns.
"Some have warned, with some justification, the reality could be catastrophic for settlements such as Ashurst, Bramber, Cowfold, Henfield, Partridge Green, Small Dole, Steyning, Twineham, Upper Beeding, Wineham and Woodmancote among others."
The West Burgess Hill Company has approached West Sussex County Council about the development, which would also include a business complex.
Mr Maidment said residents feared a flooding scenario like those seen in Chichester or Lewes.
He said: "Burgess Hill has been the fastest growing town in Europe for the last six years." He said the proposed development could render many properties downstream uninsurable.
Keith Nethercoate-Bryant, chairman of the emergency committee for Upper Beeding Parish Council, said: "The consequence of this proposed development west of Burgess Hill will be beyond comprehension for Upper Beeding and other settlements on the flood plain."
Bryn Thomas, chairman of Steyning Parish Council, said: "The residents of Steyning are deeply concerned about developments that jeopardise the delicate balance of the Adur flood plain."
Roger Glanville, chairman of Bramber Parish Council, said: "The proposed development will potentially increase the risk of flooding in the wetlands of Bramber Parish."
Howard Flight, Tory MP for Arundel and South Downs, said a thorough risk assessment was needed.
The West Burgess Hill Company was unavailable for comment.
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